In eukaryotes, protein degradation is mediated through the ubiquitin pathway in which proteins targeted for destruction are ligated to the 76 amino acid polypeptide ubiquitin. Ubiquitinated proteins then serve as substrates for the 26S proteasome, a multicatalytic protease, which cleaves proteins into short peptides through the action of its three major proteolytic activities. Proteasome-mediated degradation plays a key role in many processes such as antigen presentation in the context of the major histocompatibility complex (MHC) class I, apoptosis and cell viability, antigen processing, NF-KB activation, and transduction of pro-inflammatory signals.
The 20S proteasome is a 700 kDa cylinder-shaped multi-catalytic protease complex comprised of 28 subunits, classified as alpha- and beta-type, that are arranged in 4 stacked heptameric rings. In yeast and other eukaryotes, 7 different subunits form the outer rings and 7 different subunits comprise the inner rings. The alpha subunits serve as binding sites for the 19S and 11S regulatory complexes, as well as a physical barrier for the inner proteolytic chamber formed by the two subunit rings. Thus, in vivo, the proteasome is believed to exist as a 26S particle. In vivo experiments have shown that inhibition of the 20S form of the proteasome can be readily correlated to inhibition of 26S proteasome.
In addition to the constitutive proteasome, which is ubiquitously expressed, there is an alternative complex, the immunoproteasome, which can be found in immune cells or in cells exposed to inflammatory cytokines, such as IFN-γ and TNF-α. The immunoproteasome differs from the constitutive proteasome in its subunit composition. It contains subunits with chymotrypsin-like (β5i/LMP7), caspase-like (β1i) and trypsin-like (β2i) protease activity, that replace their counterparts in the constitutive proteasome (β5c, β1c, and β2c respectively). When all three IFN-γ-inducible subunits are present, the proteasome is referred to as the “immunoproteasome”. Thus, eukaryotic cells can possess two forms of proteasomes in varying ratios. The immunoproteasome plays an essential role in the generation of antigenic peptide repertoire and shaping MHC class I restricted CD8+ T cell response (see Basler et al. Immunoproteasomes down-regulate presentation of a subdominant T cell epitope from lymphocytic choriomeningitis virus. J Immunol 173:3925-3934, Moebius, J., M. et al. 2010. Immunoproteasomes are essential for survival and expansion of T cells in virus-infected mice. Eur J Immunol 40:3439-3449).
LMP7/β5i is an essential subunit of the immunoproteasome. It regulates inflammatory cytokine production and immune cell functions beyond its role in the generation of MHC class I-restricted epitopes. A small molecule LMP7 inhibitor, PR-957, has been shown to potently block both human and mouse Th1/17 differentiation (see Muchamuel, T., et al. 2009. A selective inhibitor of the immunoproteasome subunit LMP7 blocks cytokine production and attenuates progression of experimental arthritis. Nat Med 15:781-787; Kalim, K. W., et al. 2012. Immunoproteasome Subunit LMP7 Deficiency and Inhibition Suppresses Th1 and Th17 but Enhances Regulatory T Cell Differentiation. J. Immunol. 189:4182-4293), B cell effector functions and production of proinflammatory cytokines (IL-6, TNF-α, IL-23) (see Basler, M., et al. 2010. Prevention of experimental colitis by a selective inhibitor of the immunoproteasome. J Immunol 185:634-641). In addition, LMP7 inhibition with PR-957 has been demonstrated to produce therapeutic benefits in several preclinical autoimmune disease models. For example, PR-957 was shown to significantly inhibit disease activity in murine collagen-induced arthritis, including significant reduction of inflammation and bone erosion (see Muchamuel, T., et al. 2009. A selective inhibitor of the immunoproteasome subunit LMP7 blocks cytokine production and attenuates progression of experimental arthritis. Nat Med 15:781-787). PR-957 also reduced plasma cell numbers and anti-dsDNA IgG levels in the MRL/lpr lupus model, and prevented disease progression. (see Ichikawa, H. T., et al. 2012. Beneficial effect of novel proteasome inhibitors in murine lupus via dual inhibition of type I interferon and autoantibody-secreting cells. Arthritis Rheum 64:493-503). In addition, PR-957 reduced inflammation and tissue destruction in a murine DSS-induced colitis model (see Basler, M., et al. 2010. Prevention of experimental colitis by a selective inhibitor of the immunoproteasome. J Immunol 185:634-641). Also, PR-957 has been shown to be efficacious in an autoantibody-driven Hashimoto's thyroiditis model (see Nagayama, Y., et al. 2012. Prophylactic and therapeutic efficacies of a selective inhibitor of the immunoproteasome for Hashimoto's thyroiditis, but not for Graves' hyperthyroidism, in mice. Clin Exp Immunol. 168:268-273). In addition, LMP7 knockout mice are protected from disease in IBD models (see Basler, M., et al. 2010. Prevention of experimental colitis by a selective inhibitor of the immunoproteasome. J Immunol. 185:634-641; Kalim, K. W., et al. 2012. Immunoproteasome Subunit LMP7 Deficiency and Inhibition Suppresses Th1 and Th17 but Enhances Regulatory T Cell Differentiation. J Immunol. 189:4182-4293; Schmidt, N., et al. 2010. Targeting the proteasome: partial inhibition of the proteasome by bortezomib or deletion of the immunosubunit LMP7 attenuates experimental colitis. Gut 59:896-906. Additionally, inhibition of LMP7 with the selective inhibitor PR-924 has been shown to inhibit growth of multiple myeloma cell lines and primary patient tumor cells, including those resistant to conventional and novel prior therapies (see Singh, A. V., et al. 2011. PR-924, a Selective Inhibitor of the Immunoproteasome Subunit LMP-7, Blocks Multiple Myeloma Cell Growth both in Vitro and in Vivo. Br J Haematol. 2011 January; 152(2): 155-163).